Inactivation of the bed nucleus of the stria terminalis suppresses the innate fear responses of rats induced by the odor of cat urine

Abstract

In this study, we investigated whether two brain regions, the bed nucleus of the stria terminalis (BNST) and the basolateral amygdala (BLA), affected male rats’ (Rattus norvigicus) ability to innately discriminate between a predator odor (cat urine) and female rat urine. Muscimol, a GABAa receptor agonist, was bilaterally microinjected into either the BNST or BLA of rats through implanted stainless-steel guide cannulas to temporarily inactivate these brain nuclei. The behavioral responses of the treated rats to female rat urine and cat urine were then tested in an experimental arena. Compared to a saline infusion control, the injection of muscimol into the BNST strongly reversed the innate aversion of rats to cat urine but the injection of muscimol into the BLA had no effect. Furthermore, intra-BNST infusion of muscimol caused rats to be equally attracted to urine from cats and female rats but intra-BLA infusion did not stop rats manifesting fear on exposure to cat urine and exploratory behavior on exposure to female rat urine. We conclude that the BNST plays a more crucial role in modulating innate fear responses in rats than the BLA.

Introduction

Innate fear of predator odor is beneficial to prey animals as this can allow them to modify their behavior before predators are encountered (Edut and Eilam, 2003).

The fundamental brain structures involved in the formation, consolidation, and retrieval of fear memories have now been identified (Fendt and Fanselow, 1999, LeDoux, 2000, Davis and Whalen, 2001, Maren, 2001, Maren and Quirk, 2004). For example, the amygdala is a candidate region for the part of the brain where fear memories are originated and stored. The basolateral complex of the amygdala (BLA; consisting of the lateral, basolateral, and basomedial nuclei) is where CS (conditional stimulus) and US (unconditional stimulus) information converge and associate (LeDoux, 1998, Davis and Whalen, 2001, Maren, 2001, Schafe et al., 2001, Fanselow and Gale, 2003). However, some studies have produced contradictory results; for example, Fendt et al. (2003) found that the amygdala was unrelated to innate freezing, whereas another study found that inactivation of the medial nuclei of the amygdale of rats blocked freezing to predator odor (Muller and Fendt, 2006).

The bed nucleus of the stria terminalis (BNST), known as the extended amygdala, is adjacent to the anterior commissure in the basal forebrain and forms the rostral part of the continuum. The BNST has been strongly implicated in mediating responses to stimuli that contain an affective salience. Several studies have suggested that the BNST mediates behavioral responses to acute and chronic aversive stimuli (Casada and Dafny, 1991, Walker and Davis, 1997). This is supported by evidence that the BNST is activated in response to stress and modulates anxiety-related behaviors in several animal models. More specifically, BNST inactivation has been found to decrease the neuroendocrine and behavioral responses to stress (Gray et al., 1993), and to block some forms of unconditioned fear (Walker and Davis, 1997, Davis et al., 1997).

In the laboratory, the odors most frequently employed to elicit fear responses in rodents include cat odor and trimethylthiazoline (TMT), a synthetic compound isolated from fox feces (Maury et al., 1984). Rats exposed to either cat odor or TMT often display a variety of behavioral and physiological responses indicative of fear, including standing still, avoidance, and increased secretion of stress hormones. However, rodents appear to engage in risk assessment more frequently in tests involving cat odor (Holmes and Galea, 2002, Hebb et al., 2004) than TMT (Rosen, 2004). Cat urine has been widely used in stress experiments on rodents. Different studies have, however, reported contradictory results; for example, Bramley et al. (2000) found that the odor of cat urine strongly blocked the motor activity of Kapiti rats, while Blanchard et al. (2003) found that the cat urine odor could not induce rats to stand still. In the present study, we took female rat urine odor and cat urine odor as the two quite opposite social odor stimuli; the paired odors may provide a higher sensitivity for social recognition. We investigated whether inactivation of the BLA or BNST in rat blocks the fear responses induced by the cat urine stimulus.